http://www.emfnews.org cell tower systems.
How do Wireless Telephone Systems Work?
Wireless telephone systems work on a different concept from radio transmission systems most people are familiar with, such as television and radio stations. In most types of radio transmission systems, the object is to transmit your signal as far as possible, in order to maximize the amount of listeners or viewers you may have.
In cellular systems, the object is to transmit a controlled signal. This is done to maximize the amount of channels that are given to each cellular provider for use. Cellular systems are assigned a set number of channels for a given area, usually around 400. In order to maximize the amount of calls/channels per given geographic area, they break the coverage area into a series of cells. Each cell can cover anywhere from a one mile radius from the base station in the city and urban areas to a 10 mile radius in the countryside and rural areas. Usually an arrangement of seven repeating cells is used, with 50 or 60 channels used per cell. As you move a mobile phone between these cells, the mobile phone is 'handed off' between the cell sites and channels, being controlled by the mobile telephone switching office (MTSO), or mobile switching center (MSC). This makes use of the most important part of the cellular system, frequency re-use. There are only a limited amount of channels available in any cellular system, and this system makes the same channel available in different geographic parts of the system, to different users.
Signals are sent back and forth between the MTSO and the cell site over high capacity circuits (DS3) or microwave links. This is known as backhaul, or the process of bringing the signal from the switch to and from the individual cell site.
There are two main frequencies used for wireless telephone communications in the United States. These are 1.9 Ghz (1900 Mhz) and 800 Mhz. All systems using 1.9 Ghz utilize digital technology and are referred to as PCS http://www.emfnews.org/headset.html (less info)